(Prior Art Statement)
Representative of the closest known prior art is U.S. Pat. No. 3,435,365 to J. C. Greeson, Jr., filed Oct. 1, 1965, issued Mar. 25, 1969, entitled "Monolithically Fabricated Operational Amplifier Device with Self-Drive", U.S. Pat. No. 3,551,836 to J. C. Greeson, Jr., filed Jan. 17, 1968, issued Dec. 29, 1970, entitled "Differential Amplifier Circuit Adapted for Monolithic Fabrication", and an article entitled "Amplifier with Automatic Gain Control" by H. L. Funk in the IBM Technical Disclosure Bulletin, Vol. 6, No. 3, pages 70-71 (August 1963).
The application of differential amplifier circuits for use as cathode ray tube (CRT) video amplifiers is well known in the prior art. In high speed operation of these amplifiers a large voltage swing, because of short dot times, has been desirable. However, a low output impedance inherent in the transistor amplifier circuitry coupled with the desirable large voltage swing translates to high power dissipation of the active output stages.
High power dissipation of an amplifier requires heat sinking. The heat sinking apparatus causes problems of unwanted capacitance (which reduces the speed of the circuitry) as well as additional costs and circuit packaging volume.
Although the power dissipated in a saturated portion of a differential amplifier is somewhat reduced from that dissipated in a similar portion of a differential amplifier circuit operated in a linear mode, the saturated mode leads to unacceptable delays in turning off this section of the differential amplifier circuit.
If the maximum current that can be conducted in either leg of the differential amplifier is too small the voltage swing of the amplifier circuit will be unacceptably low. Accordingly, for maximum voltage swing, optimum speed, and minimum power dissipation an optimum maximum current can be defined at which it is always desirable for one of the two legs of the differential amplifier circuit to conduct. This current is high enough to ensure adequate voltage swing but does not result in saturation of either output transistor which would have the undesirable result of time delays in switching and, therefore, decreased amplifier bandwidth. This optimum current, however, is substantially higher than that current at which maximum dissipation in the differential amplifier output transistors would occur.
It has, of course, been known in the prior art to employ closely regulated power supplies for a differential amplifier to achieve a stable set of desirable operating characteristics over the long term. However, with the present differential amplifier circuit it was recognized that significant power supply variations may occur. Thus, it was impossible to achieve with predictability a maximum optimum current at which the differential amplifier is to operate. It has, therefore, been apparent that to achieve this optimum current flow through the output devices of a differential amplifier under varying power supply conditions some type of automatic biasing arrangement is desirable.
A form of biasing that has heretofore been employed to control the current flow through the output devices of a differential amplifier is a variable current source connected between the junction of the two output devices and a voltage source. In the case of NPN transistors in the differential amplifier output stages, this current source is interposed between the junction of the emitters of these transistors and a negative voltage supply.
Each of the prior art references shows an arrangement of a variable current source connected between the junction of the emitters of the differential amplifier output transistors (NPN) and a negative voltage supply although in none of these references is the current source varied in accordance with this invention to set a constant optimum current. In U.S. Pat. No. 3,435,365 a portion of the output current is used to control the variable current source interposed between the emitters of the output transistors and a negative voltage source. As the output signal increases this current feedback causes a decrease in the current through the amplifiers so that an automatic gain function is realized. U.S. Pat. No. 3,551,836 is similar to U.S. Pat. No. 3,435,365 in that the total amplifier output current is sampled and is used in a feedback path to vary the amplifier gain. Also, the IBM Technical Disclosure Bulletin article of H. L. Funk describes another automatic gain control function implemented by using a variable current source between a junction of the emitters in an NPN differential amplifier and a negative voltage source. In the Funk circuit the amplifier output voltage is rectified and integrated to control the variable current source. In none of these prior art examples is the output voltage at each of the differential amplifier output transistors sampled and compared to a reference voltage or a base bias voltage of these transistors to maintain an optimum current flow through the amplifier to optimize the performance of the output transistors, keeping them out of saturation while otherwise maintaining a substantially low power dissipation.